ahci: fix signature generation

ahci: fix signature generation

The initial register device-to-host FIS no longer needs to specially
set certain fields, as these can be handled generically by setting those
fields explicitly with the signatures we want at port reset time.

(1) Signatures are decomposed into their four component registers and
    set upon (AHCI) port reset.
(2) the signature cache register is no longer set manually per-each
    device type, but instead just once during ahci_init_d2h.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1441140641-17631-3-git-send-email-jsnow@redhat.com